The subject matter disclosed herein relates to monitoring the condition of rotating machinery, such as electric motors, and, more specifically, to monitoring the condition of rotating machinery during operation at either constant or variable speed using a position sensor.
Because a rotating machine repeatedly travels over a closed, rotational path, the machine will repeatedly encounter any problem that develops in that path. For example, if a portion of a bearing in a motor becomes worn, the rotor will repeatedly travel over the worn surface. Many such conditions establish undesirable vibrations in the rotating machine. The vibrations may, in turn, accelerate failure of the worn, or another, component of the rotating machine. In addition, certain types of faults develop vibrations having unique and identifiable frequency components. Thus, it is desirable to monitor vibrations in rotating machine to monitor overall performance of the machine and to identify certain failure conditions.
Historically, it was known to mount a sensing device, such as an accelerometer, to the outside of the rotating machine. The accelerometer provides a signal corresponding to the vibrations in the machine. However, the use of accelerometers has various disadvantages. For example, vibrations may be present in one plane and not another. Consequently, it is often necessary to mount multiple accelerometers on the rotating machine to measure, for example, horizontal or vertical vibrations. In addition, the accelerometer is typically not utilized for control of the rotating machine and is, therefore, not included on the machine. Consequently, using accelerometers to monitor performance of the rotating machine typically creates an additional expense and requires on-site installation to identify problems after they arise in the field. Thus, it would be desirable to monitor vibration in the machine without requiring additional sensors.
Another challenge faced when performing vibration analysis is that the rotating machine must typically be operated at a constant speed to perform the vibration analysis. However, many rotating machines perform under variable speed operation, and it may be necessary to include a diagnostic mode of operation in the machine controller to allow constant speed operation. Thus, it would also be desirable to be able to monitor vibration in the machine under variable speed operation.